feat: organize Copilot engineering agents

[placerda]

Purpose

Reorganize the existing GPT-RAG GitHub Copilot guidance into a stable global contract, scoped instructions, specialized engineering agents, and reusable skills.

Changes

• preserves the existing GPT-RAG branching, release, changelog, and documentation policy
• keeps the GPT-RAG project purpose, architecture boundaries, security posture, and multi-repository model in AGENTS.md
• adds architecture, implementation, and release engineering agents
• adds engineering, ADR, documentation consistency, and multi-repository release skills
• adds scoped instructions for Python configuration, lifecycle hooks, contracts, and release surfaces
• validates agent, skill, and instruction YAML frontmatter and local references in CI

Scope

This change affects GitHub Copilot engineering guidance only. It does not change GPT-RAG runtime agents, deployment behavior, configuration, or product documentation.

Validation

• python .github/scripts/validate-agentic-assets.py
• Python compilation for the validator
• workflow YAML parsing
• git diff --check